Cardiopulmonary resuscitation (CPR) can cause intrathoracic airway closure and a drop in lung volume below the functional residual capacity. But excessive insufflation may cause chest distension and put blood flow at risk. Here, researchers predict that thoracic distension can be reflected in the capnogram (CO2 signal) recorded during continuous chest compressions. This study aims to determine whether thoracic distension can be detected using a capnogram during CPR and determine the effects of thoracic distention on gas exchange and hemodynamics. Investigators found 3 distinct patterns on capnograms from patients who experienced out-of-hospital cardiac arrest: intrathoracic airway closure, thoracic distension, and a regular pattern. For automatic recognition, an algorithm was developed. They replicated CO2 patterns in human cadavers, evaluated the effect of tidal volume and respiratory mechanics on thoracic distension using a mechanical lung model and investigated the effect of thoracic distension patterns on several circulation parameters in a pig model of CPR. Notably, 202 capnograms were gathered as part of the clinical data (1). About 35% had intrathoracic airway closure, 22% had thoracic distension, and 43%  showed a regular pattern. Hypotheses and Tests, Part 2: Thoracic distension CO2 patterns were replicated in 5 cadavers when higher insufflated volumes were used. Higher volumes and longer time constants were related to thoracic distension patterns in the mechanical lung model. A CO2 pattern of thoracic distension was related to a substantial drop in blood pressure and cerebral perfusion in 6 pigs during CPR with varying tidal volumes but not tidal volume per se. It is possible to identify capnogram during CPR that represents intrathoracic airway closure, thoracic distension, or a regular pattern. Studies have shown that a detrimental influence of ventilation on blood pressure and cerebral perfusion during CPR is related to a thoracic distension pattern on the capnogram, which is not anticipated by tidal volume alone.